1. Field of the Invention
The present invention relates to a dark field illumination apparatus and a dark field illumination method more particularly to an epi-illumination type of dark field illumination for an object to be observed in a microscope.
2. Related Background of the Invention
In semiconductor device manufacturing steps, to inspect dusts, flaws and the like on a wafer that is a photosensitive substrate, a microscope observation using a dark field illumination has been often used. In the microscopic observation using the dark field illumination, areflection light from an object to be observed and a transmission light therefrom are not directly observed, but a scattered light from the object is observed. In the case where the object to be observed is a wafer and the like, the object can be observed with a better contrast in a dark field illumination than in a bright field illumination.
In a conventional dark field illumination apparatus, incidentally, since an optical arrangement is constructed such that an object undergoes approximately a critical illumination, illumination light is diffused so that a light beam for illumination does not form an image on a surface of the object. To be more specific, in the case where the light beam for illumination is guided into the object through a ring-shaped concave reflection mirror, the illumination light is diffused by a diffuser arranged closer to a light source than the concave reflection mirror. In this case, the light diffused by the diffuser is not incident partially onto the concave reflection mirror. In other words, a part of the light diffused by the diffuser does not contribute to the illumination, thus causing a loss of a quantity of light.
On the other hand, in the case where the light beam for illumination is guided into the object through a ring-shaped condenser lens, a diffusing film is formed on an incidence plane of the condenser lens, and the illumination light is diffused by a function of the diffusing film. In this case, a fixed diaphragm for preventing a flare is disposed in an optical path between the condenser lens and the object so that the light beam diffused by the diffusion film does not illuminate the outside of a visual field when a low magnification microscope object lens is particularly used. As a result, the part of the light passing through the condenser lens is obstructed by the fixed diaphragm and lost without contributing to the illumination. At the same time, a working distance is shortened by the disposition of the fixed diaphragm, and an operability is deteriorated.
As described above, in the conventional dark field illumination apparatus, there has been a disadvantage that a sufficient brightness cannot be obtained in an illuminated field of the object due to a loss of a quantity of light created by the diffuser or the fixed diaphragm.
Moreover, since the conventional dark field illumination apparatus does not adopt an optical arrangement for Koehler illumination, there has been a disadvantage that evenness in brightness exists in a wide visual field to be observed particularly at the time of using a low magnification microscope object lens.